1. Field of the Invention
The present invention relates to a motor vehicle steering system.
2. Description of Related Arts
In the rack-and-pinion electric power steering system disclosed in Patent Document 1 listed below, a bushing that supports an end portion of a rack shaft movably has an odd sectional shape at the inner periphery so as not to constrain the rack shaft in a direction perpendicular to the axis of the rack shaft and perpendicular to the rack pitch surface of the rack teeth. In detail, the bushing is formed into a slot whose diameter in the direction perpendicular to the rack pitch surface is larger than the diameter in the direction parallel to the rack pitch surface.
In Patent Document 1, under low load, the bushing does not constrain the rack shaft in the direction perpendicular to the rack pitch surface, and therefore, the rack shaft is supported at two points of the portion meshing with the pinion and the portion of a ball screw mechanism. Under high load such as stationary steering, the bushing constrains the rack shaft in the direction parallel to the rack pitch surface, and therefore, the rack shaft is supported at three points of the portion meshing with the pinion, the portion of the ball screw mechanism, and the bushing.
When the rack shaft is formed of a metal tube material, in order to realize an uneven thickness so that the flat portion of the rack tooth forming D-shaped section becomes thick in thickness, a method for manufacturing an uneven thickness metal tube by using eccentric drawing was proposed (for example, refer to Patent Documents 2 and 3 listed below).
In addition, a steering control device that transmits a radial load and a thrust load that are transmitted from the tire to the rack via a steering knuckle via an elastic body was proposed (for example, refer to Patent Document 4 listed below).
Further, a rack-pinion steering system in which the axis center of the support surface on one end of the rack shaft is made eccentric from the axis center of the support surface on the other end to prevent the rack shaft from twisting was proposed (for example, refer to Patent Document 5 listed below).
Further, an electric power steering system in which a guide bushing supporting the outer periphery of the rack shaft is supported on the inner periphery of a ball nut via a rolling bearing was proposed (for example, refer to Patent Document 6 listed below).
Further, an electric power steering system in which an end bushing supporting the rack shaft movably in a direction of the axis of the rack shaft is arranged so as not to constrain the rack shaft in a direction perpendicular to the axis and perpendicular to the pitch surface of the rack teeth was proposed (for example, refer to Patent Document 7 listed below).
Further, an electric actuator using a steer-by-wire ball screw was proposed (for example, refer to Patent Document 8 listed below).
Patent Document 1: Japanese Published Unexamined Patent Application No. 2006-76541 (8th to 9th paragraphs, 28th to 29th paragraphs, and 32nd to 33rd paragraphs)
Patent Document 2: Japanese Published Unexamined Patent Application No. 2010-105031 (Abstract)
Patent Document 3: Japanese Patent Publication No. 4259854
Patent Document 4: Japanese Published Examined Patent Application No. H03-56944 (FIGS. 6(a) to 6(c), line 9 of 28th column of page 14 to line 42 of 32nd column of page 16)
Patent Document 5: Japanese Published Unexamined Patent Application No. 2000-233759
Patent Document 6: Japanese Published Unexamined Patent Application No. 2010-132060
Patent Document 7: Japanese Published Unexamined Patent Application No. 2006-76541
Patent Document 8: Japanese Translation of International Application (Kohyo) No. 2007-531489
In Patent Document 1, the rack shaft is prevented from rotating by a meshing pressure between the rack teeth and the pinion teeth. The bushing of Patent Document 1 is formed into a slot, so that it increases the manufacturing cost and does not have a function to restrict the rack shaft from rotating.
On the other hand, generally, in a rack-pinion motor vehicle steering system, a section on the end portion of the rack shaft is formed to have a D shape, and the section on the inner periphery of the bushing is also formed to have a D shape, and accordingly, the rack shaft is reliably restricted from rotating. However, this increases the manufacturing cost.
In recent years, a so-called steer-by-wire motor vehicle steering system in which the mechanical joining between the steering wheel and the steered wheels is cut off is proposed. In this type of motor vehicle steering system, it is necessary to restrict the steered shaft from rotating by using the above-described bushing, etc.
On the other hand, many industrial vehicles and welfare vehicles are of a type that turns the rear wheel, and in such a vehicle, in order to secure a necessary steering angle, the rear wheels as steered wheels must be turned at a great steered angle. Therefore, the steered shaft tends to be subjected to a high radial load due to a road surface reaction force. For example, in a steer-by-wire vehicle, a ball screw mechanism provided on the steered shaft and the actuator that drives the ball screw mechanism are subjected to a high radial load and their durability is deteriorated.
An object of the present invention is to provide a motor vehicle steering system that can restrict a steered shaft from rotating inexpensively. Another object of the present invention is to provide a motor vehicle steering system capable of reducing the influence of a radial load to be applied to the steered shaft and excellent in durability.